1. Field of the Invention
This invention relates generally to the field of X-ray diffraction and, more specifically, to the analysis of single-crystal specimen using an active pixel array sensor.
2. Description of the Related Art
Single-crystal X-ray diffraction (SC-XRD) is a method for determining the three-dimensional atomic structure of a crystalline compound. A single-crystal specimen of the compound is irradiated with monochromatic X-ray radiation from different directions, some of which is diffracted in specific patterns and detected by an active pixel sensor. The structural information of the specimen is determined from the geometry and relative intensities of these diffraction patterns. The intensities are integrated from the pixels in the active pixel array sensor images.
A typical laboratory system 100 for performing single-crystal diffraction experiments normally consists of five components as shown in FIG. 1. The components include an X-ray source 102 that produces a primary X-ray beam 104 with the required radiation energy, focal spot size and intensity. X-ray optics 106 are provided to condition the primary X-ray beam 104 to a conditioned, or incident, beam 108 with the required wavelength, beam focus size, beam profile and divergence. A goniometer 110 is used to establish and manipulate geometric relationships between the incident X-ray beam 108, the crystal sample 112 and the X-ray sensor 114. The incident X-ray beam 108 strikes the crystal sample 112 and produces scattered X-rays 116 which are recorded in the sensor 114. A sample alignment and monitor assembly comprises a sample illuminator 118 that illuminates the sample 112 and a sample monitor 120, typically a video camera, which generates a video image of the sample to assist users in positioning the sample in the instrument center and monitoring the sample state and position.
The goniometer 110 allows the crystal sample 112 to be rotated around several axes. Precise crystallography requires that the sample crystal 112 be aligned to the center of the goniometer 110 and maintained in that center when rotated around the goniometer rotational axes during data collection. During exposure, the sample (a single crystal of the compound of interest) is rotated in the X-ray beam 108 through a precise angular range with a precise angular velocity. The purpose of this rotation is to predictably bring Bragg reflections into constructive interference with the incident beam 108. During this time, called the charge integration time, the pixels of the sensor receive and integrate the X-ray signals.
Active pixel array sensors used in SC-XRD may include CMOS or CCD imagers. While effective, sensors such as these are often subject to pixel defects. The affected pixels may be permanently dark (i.e., “dead pixels”), permanently bright (i.e., “hot pixels”), or they may exhibit other behavior that prevents an accurate signal from being detected at these pixel locations. As such, to maintain an accurate signal detection, diffraction intensities that overlap with defective pixels must either be rejected, or estimated values must be in place of a useful response from the defective pixels.
Most established methods for pixel defect correction use nearby good pixels to determine replacement pixel values. In the simplest case, the replacement pixel value is copied from one of the neighboring pixels. More sophisticated approaches use linear or higher order interpolation across neighbors on both sides of the defective pixels and in one or two dimensions. In most cases the corrected pixel values are good enough to be inconspicuous to the human eye but, in the case of single-crystal X-ray diffraction images, they seldom pass the scrutiny of a numerical analysis. The affected intensities are often trapped as outliers and need to be rejected from the data. Moreover, if they remain undetected, they can negatively influence the result of the structure analysis.